A scanning probe microscope (SPM) is a type of microscope that forms images of a specimen using a physical probe that scans over the surface of the specimen. The scanned probe may react with the specimen through a variety of physical forces, including mechanical contact forces, van der Waals forces, capillary forces, chemical bonding forces, electrostatic forces, and magnetic forces. SPMs measure different forces to determine different properties of the specimen, and display the sample properties on an image.
Types of SPMs include the scanning tunneling microscope (STM), which measures conducting sample, and the atomic force microscope (AFM), which can measure various properties of non-conductive sample. AFMs are well known and are described, for example, in U.S. Pat. No. 6,185,991 to Hong et al. for “Method and Apparatus for Measuring Mechanical and Electrical Characteristics of a Surface Using Electrostatic Force Modulation Microscopy Which Operates in Contact Mode”, which is hereby incorporated by reference. An AFM can operate in a contact mode, a tapping mode, or a non-contact mode.
FIG. 1 shows an AFM 100 that includes a probe tip 102 at the distal end of a cantilever 104. A positioner 106, typically comprising piezoelectric actuators, scans the cantilever 104 with the probe tip 102 across the surface of a sample 108, which may include features, such as a nanoscale structure 110. Cantilever 104 acts like a spring. When cantilever 104 is deflected and released, it oscillates at a resonant frequency.
Forces between the sample 108 and the cantilever 104 with probe tip 102 cause a deflection of cantilever 104. A laser 116 directs a beam of light 118 towards a reflective surface on cantilever 104 near tip 102, and the reflected light 120 is detected by a position sensitive photon detector 122, which produces an electrical output signal corresponding to the position of the tip. The output signal from the detector 122 is processed by a signal processor 124 to determine the deflection of tip 102 over time. The cantilever oscillation, and therefore the signal output from photon detector 122, is essential sinusoidal and characterized by a frequency, amplitude, and phase. The various forces between the probe and the sample will affect these sinusoidal properties. Signal processor 124 may include one or more lock-in amplifiers 126 to extract signals corresponding to specific frequencies from other signals and noise present in the output signal from detector 122. In various applications, the amplitude, frequency, and/or phase of the cantilever vibration are detected and used to determine a local property of the sample.
A controller 130 controls AFM 100 in accordance with instructions input through user interface 132 or in accordance with program instructions stored in computer memory 134. Controller 130 also controls an imaging device 136, such as a computer display screen, to display sample images formed by AFM 100. In some applications, controller 130 uses the tip deflection to provide feedback to positioner 106 to raise or lower the cantilever 104 to maintain a constant distance between the probe tip 102 and the sample surface. By “distance between the probe tip and the sample surface” is meant the distance from the local sample surface below the probe to the rest position of the probe. In other applications, the probe is scanned in a straight line, and so the height of the probe about the sample surface varies as the local surface topography.
A sample voltage source 140 can apply a dc bias voltage, an ac voltage, or a combination of both to sample 108. As used herein, applying a dc bias voltage may include applying a zero voltage, that is, grounding an element. A scanning probe voltage source 142 can apply a dc bias voltage, an ac voltage, or both to tip 102.
When the AFM is being used to measure voltages, the sample 108 is optionally positioned within a guard chuck 144, which secures the sample and partly surrounds it with a conductive material to reduce stray electrical potentials that can affect the electrical measurements. The potential from sample voltage source 140 can be applied to the bulk sample through the guard chuck, or through a conventional chuck. The potential can also be applied to contact pads on the sample.
When an AFM is operated in a mode to detect electrostatic force, it is referred to as an electrostatic force microscope (EFM). The EFM is a type of vibrating, non-contact AFM in which a force generated by applying an electrical potential difference between the probe tip and the sample is measured. An EFM is described, for example, in P. Girard, “Electrostatic Force Microscopy: Principles and Some Applications to Semiconductors,” Nanotechnology 12, 485 (2001).
As described in Girard, a voltage difference between a sample and an AFM tip creates a force proportional to the change in capacitance with probe height and the square of the potential difference:
  F  =            1      2        ⁢          dC      dz        ⁢          V      2      
The voltage, V, is a combination of any applied dc voltage (Vdc), applied sinusoidal voltages (VAC), the contact potential (Vcp), and any externally induced surface voltage (Vinduced).V=(Vcp+Vdc+Vinduced)+VAC sinΩt 
The force can be decomposed into three frequency terms. A dc term:
      F          d      ⁢                          ⁢      c        =            1      2        ⁢                  dC        dz            ⁡              [                                            (                                                V                                      d                    ⁢                                                                                  ⁢                    c                                                  +                                  V                  cp                                +                                  V                  induced                                            )                        2                    +                                    1              2                        ⁢                          V                              A                ⁢                                                                  ⁢                C                                                    ]            corresponds to a continuous bend of the cantilever, which is hard to detect.
A frequency Ω term:
      F    Ω    =            dC      dz        ⁢          (                        V                      d            ⁢                                                  ⁢            c                          +                  V          cp                +                  V          induced                    )        ⁢          V              A        ⁢                                  ⁢        C              ⁢    sin    ⁢                  ⁢    Ω    ⁢                  ⁢    t  
is dependent on the capacitive coupling and the sample voltages Vcp and Vinduced and is therefore useful to show voltage contrast on the sample. In some implementations, a feedback loop maintains FΩ at 0 by making Vdc equal to =−(Vcp+Vinduced), which can improve image quality under some conditions.
A frequency 2Ω term:
      F          2      ⁢      Ω        =            1      4        ⁢          dC      dz        ⁢          V              A        ⁢                                  ⁢        C            2        ⁢    cos    ⁢                  ⁢    2    ⁢                  ⁢    Ω    ⁢                  ⁢    t  is dependent on the local capacitive coupling. A lock-in amplifier can be used to extract the FΩ or the F2Ω signal from noise when scanning at a constant tip-sample distance.
EFM is performed by scanning a probe across the sample, while applying one or more potentials to probe and/or to the sample. The applied potentials may be ac, dc, or combinations thereof.
AFM 100 of FIG. 1 shows a potential V1 applied to the probe tip 102 and a potential V2 is applied to the sample 108. While a user may desire to apply voltage V2 to the nanoscale structure 110, various factors affect the electrical properties at nanoscale structure 110, which results in intermediate potentials on nanometer scale structure 110.
The capacitance between the AFM probe tip apex and nearby region of the sample is a significant component of the EFM technique. However, parasitic capacitance from the entire sample to the AFM probe tip cone and cantilever creates additional measured force. Furthermore, in complex samples, like an integrated circuit, the nanometer scale structures at or near the surface may have potentials, which are less clearly defined due to junctions and resistance along multiple paths to the driven substrate body potential. The effect of the parasitic capacitances and the loosely constrained set of potentials creates scanned images with poor signal to noise and unclear sources of the resulting potential map.
FIG. 2 is a vibrating non-contact AFM topography image of a four micrometer by four micrometer area of an integrated circuit at the second metal level above the transistors fabricated by 22 nm process technology. A 22 nm process typically has a metal line spacing of less than 100 nm FIG. 2 shows multiple conductive lines 202 separated by insulating areas 204. In the false color image of FIG. 2, lighter colors indicate a higher elevation of the sample surface. As the magnitude of the oscillation changes with the surface topography during the scan, feedback is used to raise and lower the probe to maintain the oscillation amplitude. The drive voltage to the piezo actuator is plotted to create the topography image.
The image in FIG. 3 is a potential map formed by an EFM of the same area as shown in FIG. 2. The potential map was formed using an AFM probe scanned at a constant height. In the false color image of FIG. 3, regions of higher electrostatic potential are shown as green and regions of lower electrostatic potential are shown in blue. The brighter the color, the higher the electrostatic potential. The imaging probe tip was biased with a time varying frequency near to that cantilever's resonant frequency. FIG. 3 shows some of the metal lines 302 as regions of high electrostatic potential, but their connection paths to each other and the substrate is unclear. The image of the metal lines is unclear because, as described above, there are parasitic capacitances that affect the sensed potential and the electrical potential applied to the sample body through the chuck is modified differently before reaching each of the individual conductive lines due to junctions and different resistances along different paths to the conductors.
As semiconductor circuits get smaller and the metal lines get closer together. The “pitch,” or distance between metal lines in modern integrated circuits varies with the fabrication process, but can be typically less than 250 nm, less than 100 nm, less than 80 nm, and even less than 40 nm. As the pitch becomes smaller in each new generation of fabrication processes, it becomes more difficult or impossible to differentiate a single metal line in an EFM image. A method and apparatus is needed to image individual metal lines and sub-surface lines, to show fabrication defects, such as shorts and opens.
Tsunemi et al, “Development of dual-probe atomic force microscopy system using optical beam deflection sensors with obliquely incident laser beams,” Review of Scientific Instruments 82, 033708 (2011) describes a dual probe AFM system and uses the system to perform a Kelvin Force Measurement to determine the surface potential of a dendritic island of an α-sexithiophene thin film on a highly-doped Si substrate with a 300 nm-thick SiO2 layer. One probe was scanned with the tip-sample distance regulated by an FM (frequency modulation) detection method, while electrical charges were simultaneously injected into the a-sexithiophene thin film by another probe.